I don't speak german, but it would seem it's basic bank transfer common over here. You should get his bank information, account numbet etc. thru that "Überweisung Plus" link and then type in the required fields on your web bank.
As this is international payment, there isn't necessarely all required information to make succesful payment. I'm not sure but I think you need to get at least IBAN (International Bank Account Number) - so ask him to email those to you. Because the complexity of international transfer, it is sometimes easiest to go your bank, ask them what information is required for succesful transfer and make it there. Be sure to mark that you pay all the costs, since they could easily eat up the whole 20$. After he sees the money on his account, the memory should be on it's way.
